1. Field of the Invention
The present invention relates to piezoelectric elements and particularly energy-confinement piezoelectric elements using a thickness vibration mode.
2. Description of the Related Art
Energy-confinement piezoelectric elements fall into two primary categories: those which excite fundamental frequency thickness vibrations in the piezoelectric bodies in the area between the vibration electrodes and those which excite higher-order (harmonic) thickness vibrations in this area. Piezoelectric elements which excite fundamental thickness vibrations will only operate when piezoelectric materials having relatively high Poisson""s ratios are used. This is not the case with piezoelectric elements which excite harmonic thickness vibrations. Such piezoelectric elements are not substantially affected by the Poisson""s ratio, and do not suffer this drawback.
In general, thermally stable materials such as lead titanate have Poisson""s ratios of less than one-third which do not achieve frequency-reducing energy confinement of the fundamental thickness vibration. Such materials can be used, however, in piezoelectric elements which excite harmonic thickness vibrations, and this has attracted attention as a technology for achieving high-performance piezoelectric elements.
However, the energy-confinement elements exciting thickness vibration harmonic waves have different optimized electrode structures depending on the piezoelectric material being used, with the result that the optimum geometry of the piezoelectric element must be determined as a function of the material being used.
In an energy-confinement element, it is known that the resonant frequency of a spurious vibration called an anharmonic overtone is present near the primary harmonic resonant frequency. The anharmonic overtone is not excited when the electrode diameter, that is, the energy-confinement region, is reduced. Since the maximum electrode diameter not causing the anharmonic overtone excitation depends on the piezoelectric material being used, the maximum electrode diameter must be separately determined for each material. A non-oriented layered bismuth-based ceramic material is thermally stable and has a small electromechanical coupling coefficient, and thus, it is expected that it could be used as a high-performance oscillator having a narrow allowable error. However, the maximum electrode diameter thereof which will not excite the undesirable anharmonic overtone has not been determined.
It is an object of the present invention to provide a thermally stable piezoelectric element in which a high-performance oscillator having a narrow allowable error can be achieved.
In accordance with the present invention, a piezoelectric element includes a plurality of piezoelectric layers comprising a piezoelectric material comprising Sr, Bi, Ti, and O, at least three vibration electrodes opposing each other, each disposed among the piezoelectric layers, and an energy-confining region formed in a region in which the vibration electrodes overlap. The energy-confining region being parallel to the planes of the vibration electrodes and excites an n-th harmonic longitudinal thickness vibration. The maximum length L of a secant between two intersections on the periphery of the energy-confining region and the distance t between the topmost vibration electrode and the bottommost vibration electrode satisfy the relationship nL/t of  less than  than 10, wherein n is an integer greater than 1.
Preferably, the piezoelectric material comprises SrBi4Ti4O15.
Preferably, the topmost vibration electrode and the bottommost vibration electrode are formed on the outer surfaces of the piezoelectric layers.
Alternatively, the piezoelectric material comprises Ca, Bi, Ti, and O, and the value of the ratio nL/t is less than 9. In such a case, the piezoelectric material preferably comprises CaBi4Ti4O15.
Alternatively, the piezoelectric material comprises Sr, Bi, Nb, and O, and the value of the ratio nL/t is less than 10. In such a case, the piezoelectric material preferably comprises SrBi2Nb2O9.